JPH0215302B2 - - Google Patents

Abstract

A continuously cast strand or ingot of metal is cooled as it emerges from the mold by applying a coolant directly to the surface of the strand. In order to prevent distortion at the foot of the ingot due to too abrupt cooling, polymers of molecular weight 104-108 are added to the coolant at least during the initial phase of casting. By the addition of polymers to the coolant the kinetics of formation and discharge of vapor bubbles as the coolant strikes the ingot surface are significantly altered due to the reduction in the surface tension of the coolant; as a result of this an insulating film of coolant vapor forms on the surface of the ingot thus reducing the heat flow from the ingot.

Description

[Detailed description of the invention]

The present invention relates to a method for cooling a metal strand emerging from a mold during continuous casting, in which a cooling liquid is applied directly to the surface of the strand. Continuous casting with direct cooling is carried out in such a way that the surface of the metal strand exiting the mold is sprayed with a cooling liquid to remove heat from the metal directly below the mold. During the start-up phase of billet casting (often referred to as drop), the coolant initially impinges only on the dummy base. The resulting indirect cooling at this stage results in a gentle solidification of the molten metal, resulting in flat-sided slab legs. As the dummy base descends further, the coolant impinges directly on the slab surface and heat removal from the metal strands increases rapidly. If the thermal strain produced as a result of this thermal shock becomes greater than the yield point of the slab, permanent deformation in a convex curved shape will occur at the slab leg. Moreover, if the tensile strength of the material is exceeded, cracks will occur in the slab. In order to produce continuously cast slabs with flat bases or legs, the strand must therefore not be cooled too strongly during the start-up phase. It is known to supply coolant in a pulsating manner to reduce the cooling intensity at least during the start-up phase. It is also known that the cooling liquid, at least during the start-up phase, contains a gas introduced under pressure. When the coolant hits the surface of the slab, the gas dissolved in the coolant forms an insulating film that helps remove heat and thus reduces the cooling effect. In view of the above, the object of the present invention is to develop a method of the type mentioned at the outset, which is simple and does not suffer from the disadvantages mentioned above, and also in such a way that the cooling intensity can be reduced. The purpose is to reduce the molecular weight at least at the start-up stage of casting.
This is achieved by the method of the invention in which 10 ⁴ to 10 ⁵ polymers are added to the coolant. By adding this polymer to the coolant, the movement of vapor bubble formation and release when the coolant hits the hot slab surface is rapidly reduced as a result of a significant reduction in the surface tension of the coolant. This causes an insulating film of cooling liquid vapor to form on the surface of the slab, which impedes heat removal from the slab. The polymer is fed in concentrated form, for example as a solution of 10 to 50 g of polymer per liter of coolant, from the storage tank to the coolant supply line by means of a controlled feed pump. When water is used as the coolant, the molecular weight is 10 ⁵
~5×10 ⁶ nonionic dissolved polyethylene oxide has been found to be a particularly suitable polymer for this purpose. In a further advantageous embodiment of the invention, when water is used as the cooling liquid, the additive has a molecular weight of 10 ⁶ to
A polymer of 5×10 ⁷ partially hydrolyzed anionic polyacrylamide may be used. The polyacrylamides recommended in this case are those exhibiting a degree of hydration of 10-20% and a molecular weight of about 1.5×10 ⁷ . The method according to the invention can be carried out both in conventional molds and in electromagnetic molds for continuous casting and is particularly suitable for casting light metals, especially aluminum and aluminum alloys. The concentration of additives in the coolant medium is selected depending on the required reduction in cooling strength, but is usually on the order of 1 to 100 milligrams per liter. After the start-up phase is over, the addition of polymer to the coolant may be stopped. As another expression of the method according to the invention, the concentration of polymer in the coolant may be continuously reduced during the start-up phase. However, in some cases it may be advantageous to use the method according to the invention throughout the entire casting period. Further advantages, features and details of the invention are set out in the following description of preferred embodiments. Alloy 3004 has a cross-section of 500mm x 500mm under normal casting conditions in a vertical DC casting machine combined with an electromagnetic mold.
It was cast in the form of a 1600mm slab. The coolant feed rate was kept constant at 600 l/min during the entire casting time. The polymers listed in the table are
It was added to the cooling water until it was cast to a length of 100mm. 10-50g per liter of water by the end of this
A solution containing the polymer was pumped directly from the storage tank into the main cooling supply pipe in a controlled manner. The concentration of polymer made up in the cooling water is also shown in the same table. During the subsequent drop (casting of the billet) no polymer was added to the cooling water.

Table: By maintaining the indicated concentrations of additives in the cooling water during the start-up phase, nearly crack-free and crack-free slabs were obtained as a result of reduced cooling strength.

Claims (1)

[Claims] 1. A method for cooling a continuously cast metal strand or slab by applying a cooling liquid directly to the surface of the strand or slab as it emerges from a mold, 10 ⁴ A method for cooling continuous casting strands of metal during casting, characterized in that a polymer having a molecular weight of ~10 ⁸ is added to the cooling liquid at least at the beginning of casting. 2 Water is used as cooling water, and as a polymer
2. Process according to claim 1, characterized in that a non-ionic polyethylene oxide having a molecular weight of from 10 ^<5> to 5*10 ^<6> is used in solution. 3 Using water as the coolant and 10 ⁶ as the polymer
3. Process according to claim 2, characterized in that a partially hydrolysed anionic polyacrylamide having a molecular weight of ˜5×10 ⁷ is used. 4 Has a degree of hydrolysis of 10 to 20% and approximately 1.5×
4. Process according to claim 3, characterized in that a polyacrylamide with a molecular weight of 10 ⁷ is used. 5. characterized in that the polymer is added to the cooling liquid to a concentration of 1 to 100 milligrams per liter,
A method according to any one of claims 1 to 4. 6. Add the polymer to the cooling liquid and add the polymer to the cooling liquid.
6. A method according to claim 1, characterized in that it is supplied in the required amount in the form of a concentrated solution containing 50 mg/liter.